Iran remains the world's leading state sponsor of terrorism with funding networks and operational cells working around the world, a US government report said.

The State Department's annual survey of global terrorism accused Iran of intensifying numerous conflicts and trying to disrupt governments in the Middle East and elsewhere.

Nathan Sales, the department's coordinator for counterterrorism, said Iran-linked fundraising networks in West Africa, weapons caches in South America, and operational activity in Europe show the Islamic Republic "has no reservations about using [terrorism] on any continent".

"Iran remains the world's leading state sponsor of terrorism and is responsible for intensifying multiple conflicts and undermining US interests in Syria, Yemen, Iraq, Bahrain, Afghanistan and Lebanon," Sales told reporters on Wednesday.

The United States named Iran as a state sponsor of terror in 1984. Other countries currently on the US list include North Korea, Syria, and Sudan.

Iran maintains that its activities support oppressed people throughout the Middle East. The country is open in its assistance to groups such as Israel's adversary Hezbollah in Lebanon and Shia militias in Iraq.

The report specifically cited the activities of Iranian General Qassem Suleimani, leader of the Revolutionary Guard, who helped organise Iraqi militias against the Islamic State of Iraq and the Levant group (ISIL, also known as ISIS).

'More complex'

Worldwide attacks dropped by 23 percent last year compared with 2016, the report said. The decrease was largely attributed to gains made against ISIL in Iraq.

The number of violent incidents also declined in 2016 from the previous year.

ISIL lost territory in Syria and Iraq and was responsible for 53 percent fewer deaths than the previous year, but it still carried out attacks in 20 countries in 2017, it said.

Despite the decrease in violence, the report described threats of attack against the US and its allies around the world as "more complex".

ISIL, al-Qaeda, and similar groups have decentralised and are adopting new technologies such as simple chemical weapons and small drone systems, according to the Justice Department.

"They have become more dispersed and clandestine, turning to the internet to inspire attacks by distant followers and, as a result, have made themselves less susceptible to conventional military action," the report said.